When talking about morality, it is natural to think that there exist certain moral truths. That is, when it comes to morality, some things are true, and some things ae false, and that's all there is to it.
Surely, we think, it's true that murder is wrong. We think, more specifically, that it is a fact that murder is wrong. We think that these kind of truths are amongst the most undeniable and secure truths that we have.
There are, however, a number of problems when it comes to moral truths:
First, there is the metaphysical problem - what could a moral truth even be? What would a moral fact look like? What sort of entity is a moral truth?
There are even more troubling metaphysical questions, such as - why are moral truths intrinsically motivating or normative? Just by knowing them, we seem to be motivated to act in a certain way. There don't seem to be many other types of truths, or facts that work in this way.
Then, we have the epistemological problem; how on earth can we know what these moral truths are? What faculty do we use to know them?
These problems shed serious doubt on our initial assumption that moral truths exist. Moral truths are both metaphysically and epistemologically highly problematic.